An inverter device which is a typical example of a power conversion device includes a power conversion device main body, such as is shown in FIG. 4, as a main portion which drives a load M such as a three-phase alternating current motor. The power conversion device main body is realized as a power semiconductor module (IPM) 10 packaged including six semiconductor switching elements Q1 to Q6 formed of, for example, IGBTs (insulated-gate bipolar transistors). This kind of inverter device supplies power to the load M by the semiconductor switching elements Q1 to Q6 being interrelatedly on/off driven by an unshown control circuit, as introduced in detail in, for example, PTL 1.
The semiconductor switching elements Q1 to Q6 in the power semiconductor module (IPM) 10, by being connected in series in pairs of two, form three sets of half bridge circuits HB. Also, six free wheeling diodes D1 to D6 are connected in reverse parallel to the respective semiconductor switching elements Q1 to Q6. The three sets of half bridge circuits HB, by being connected in parallel, form a three-phase full bridge circuit which drives the load M. A1 to A6 in FIG. 4 are drive circuits which on/off drive the respective semiconductor switching elements Q1 to Q6.
The three sets of half bridge circuits HB, by being interrelatedly driven by the control circuit, supply three-phase (U-phase, V-phase, and W-phase) currents, which are 120° different in phase from one another, to the load M from the respective middle points of the half bridge circuits HB. Herein, the middle points of the half bridge circuits HB refer to the series connection point of the semiconductor switching elements Q1 and Q4, the series connection point of the semiconductor switching elements Q2 and Q5, and the series connection point of the semiconductor switching elements Q3 and Q6.
Specifically, the upper-arm semiconductor switching elements Q1, Q2, and Q3 and lower-arm semiconductor switching elements Q4, Q5, and Q6 of the half-bridge circuits HB are on/off driven in accordance with a pulse-width modulated control signal of a constant cycle, as shown in (a) of FIG. 5. Specifically, the drive circuits A1 to A6, in accordance with the control signal, generate gate drive signals and on/off drive the respective semiconductor switching elements Q4, Q5, and Q6. As a result of the on/off drive, a current corresponding to the pulse width of the control signal is supplied to the load M via the upper-arm semiconductor switching element Q1 (Q2, Q3) over a positive half-cycle, as shown in (b) of FIG. 5. Also, a current corresponding to the pulse width of the control signal is supplied to the load M via the lower-arm semiconductor switching element Q4 (Q5, Q6) over a negative half-cycle, as shown in (c) of FIG. 5.
As a result of this, an alternating current forming a sine wave is supplied to the load M from each of the half bridge circuits HB, as shown in (d) of FIG. 5. However, the current supplied to the load M is practically a pulse current synchronized with the control signal, and the pulse current forms a discrete sine-wave current waveform. FIG. 5 only shows the output current of one half bridge circuit HB, but the same also applies to the output currents of the other half bridge circuits HB, except that the phases of the half bridge circuits HB are 120° different from one another.
Meanwhile, an overcurrent protection function is provided in the drive circuit A (A1 to A6) of this kind of power conversion device, as introduced in, for example, PTL 2. The overcurrent protection function monitors a current flowing through each semiconductor switching element Q (Q1 to Q6), and when detecting an overcurrent, stops the on/off drive of the semiconductor switching element Q (Q1, Q2 to Q6).
When the semiconductor switching element Q is an IGBT, as shown in, for example, FIG. 6, the monitoring of a current flowing through the semiconductor switching element Q is carried out by detecting, via a resistor R, a current output from a current detection terminal included in the IGBT as an auxiliary emitter. Further, the overcurrent detection is carried out by a comparator 1 comparing a detection voltage obtained in the resistor R in accordance with the current and a reference voltage Vref which defines a current limit voltage. When an overcurrent is detected in this way, the operation of a driver circuit 2 which on/off drives the semiconductor switching element Q is stopped by an output of the comparator 1, thereby fulfilling the overcurrent protection function.
3 in FIG. 6 is an alarm signal generation circuit which generates and outputs an alarm signal to the control circuit when an overcurrent is detected by the comparator 1. When a current flowing through the semiconductor switching element Q, that is, a detection current Ic obtained via the auxiliary emitter exceeds the current limit value acting as an overcurrent detection threshold value, as shown in, for example, FIG. 7, the alarm signal generation circuit 3 cyclically outputs an alarm signal with a predetermined pulse width after an operation delay time of the alarm signal generation circuit 3.